1. Field of the Invention
The present invention relates generally to an optical device to be employed for optical communication system. More specifically, the invention relates to a high-brightness visible light-emitting diode (LED) to be employed as high-efficiency light source in a plastic optical fiber (POF) based optical data-link system. Such LED is also applicable for an outdoor display device, an automotive indicator and so forth.
2. Description of the Related Art
A light output efficiency of the conventional light-emitting diode (LED) is mainly dependent on a structure and shape of an electrode to be used for injecting the current to junction. Conventionally fabricated surface-emission type light-emitting diode has a quadrangular or circular-shaped upper electrode located at the center portion thereof. The area and shape of the upper electrode significantly influence the light output efficiency. For achieving higher light output efficiency, it is required to make the area of the electrode smaller and the area of a light-emitting surface wider. In the conventional LED, while employment of the quadrangular or circular-shaped electrode positioned at the center may facilitate fabrication, but it may cause disruption of gaussian beam pattern of an output beam. Especially, it may make the beam having the peak of the emission pattern at the circumferential edge portion of the electrode. The emission pattern attenuate according to increasing of distance from the electrode. This is caused by the fact that the current is concentrated at the center portion and not distributed at the position, distanced from the electrode. A beam intensity to be attained is directly proportional to a current density thereat. The conventional LED having wide beam pattern as set forth above cannot be employed in an optical data-link system. That is, even when a large core fiber is employed, the coupling efficiency is held low due to emission pattern.
One of typical examples of the conventional surface emission type LED is shown in FIGS. 1A, 1B and 2. FIGS. 1A and 1B show sections along an upper surface of an LED and an approximated near-field-emission. In FIGS. 1A and 1B, an n-type GaAs buffer layer 2, an n-type (Al.sub.x Ga.sub.1-x)In.sub.1-x P layer 3, an active layer 4, a p-type (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P layer 5, a current spreading layer 7 and a p-type GaAs cap layer 8 are grown sequentially on an n-type GaAs substrate 1. For a p-type contact, quadrangular, circular or cross shaped electrode 28 is employed in the prior art. As the current spreading layer 7, a thick Al.sub.0.7 Ga.sub.0.3 As layer is normally employed.
In FIG. 2, an approximated near-field-pattern of a light intensity 30 for LED having a quadrangular or circular shaped electrode located at the center, is shown. As shown, the light intensity 30 is maximum around the peripheral edge portion of the electrode and becomes lower at distance position from the electrode. This represents crowding of the current at the center and not distributing far away for the electrode. In this case, it is impossible to enhance the output efficiency as expected. In order to avoid crowding of current around the electrode, a blocking layer 31 is typically employed in front of the AlGaAs layer (current spreading layer) 7. FIG. 3 shows another example of the conventional LED having the blocking layer 31. Requirement of regrowth makes production cost of the LED higher. Also, darkness at the center portion in the case of circular electrode makes the coupling efficiency lowers even with a large core fiber. It is highly desirable to provide an LED structure which can provide high coupling efficiency and is suitable for mass scale production.
III-V semiconductor based visible LED (wavelength is in a range from 580 to 670 nm) which has quadrangular/circular shaped electrode at the center, have been disclosed in various publications and patent publications. In every case, quadrangular or circular shaped electrode provided at the center is used as upper contact for injecting the current. One of typical examples can be found in the paper of Sugawara et al., "Japan Journal of Applied Physics, Part 1, Vol. 31, No. 8, pp 2446 to 2451, 1992". In this report, a cross shaped electrode along with blocking layer were used in the surface-emission type LED.
However, the light output efficiency is still beyond its practical application. To enhance current spreading outside the contact, a thick window layer having low resistivity is required. Even with such thick window layer, the current spreading outside the contact is limited to a certain level. Thus, the light intensity is much lower than that required for practical application. However, in application, such as short distance data-link system, especially based on POF, employment of the conventional LED exhibits low coupling efficiency and thus is impractical in a POF based communication system. For POF based data link application, it would be highly desirable to design the LED which offers not only high brightness but also high coupling efficiency. In addition, it is highly desirable to reduce a fabrication cost as low as possible in order to contribute for lowering of a cost for the system.
Japanese Unexamined Patent Publication (Kokai) No. Heisei 2-174272 discloses a high brightness LED. In the disclosed art, the brightness of the LED is enhanced employing n-p-n-p structure under the contact, which assists for spreading the current outside the contact. The fabrication process of the conventional LED as disclosed in the above mentioned publication includes a first step of sequentially growing of an N-type junction layer, a P-type light emitting layer and an N-type transparent layer on an N-type semiconductor substrate, a second step of performing photo-etching for the N-type transparent layer, a third step of performing diffusion of Zn for forming a current spreading region, a fourth step of performing photo-etching for isolation of elements and polishing of the lower portion of the element, and fifth step of forming electrode on upper and lower portions of the element. The main drawback of the disclosed invention is that prior to form p-type contact, a mesa structure reaching to the active region has to be formed in a light emitting surface to define a current path. Also, a dopant, such as Zn, has to be diffused at high temperature environment. The post-growth high temperature treatment for Zn diffusion influences on performance characteristics due to degradation of the active region. As the light emitting surface with high p-doping is used, a large fraction of light (depending upon the light energy) can be absorbed in the diffused layer. In fabrication of this type of LED, several times of processes are required to cause high fabrication cost of the LED. In addition, difficulty may be encountered in fabrication of high speed LED for high capacitance caused by wide contact area.
Japanese Unexamined Patent Publication No. Heisei 3-20 190287 discloses a high brightness LED. In the disclosed conventional LED, a plurality of light emitting regions of mesa structure are arranged on a direction. Each of the light emitting regions is formed so that edges of the light emitting region perpendicular to the arranging direction thereof become forward mesa structure and remaining edges parallel to the arranging direction become reverse mesa structure. And an electrode is lead from only one of edges having forward mesa structure. In such LED array, for facilitating wire bonding, the contact was made on the upper layer following the mesa formation. In this case, the shape of the contact is varied to achieve high light output. The disclosed prior art is completely related to the LED array, and since the mesa structure is required to be formed prior to the electrode formation, there should have difficulties in formation of the electrode unless a thick contact is formed. This is not only results in increasing of the fabrication cost of the LED, but also impractical for application in fabrication of single LED.
Japanese Unexamined Patent Publication No. Heisei 5-211345 discloses an LED. In the disclosed prior art, between a p-type clad layer and a cap layer, an n-type current blocking layer is provided. In conjunction therewith, a portion to be the peripheral edge of the upper electrode in the current blocking layer is converted into p-type by diffusion or ion implantation of a p-type impurity through a portion of a light extraction surfaces on the cap layer where an upper electrode is not mounted. In such prior art, the current blocking layer, the type of which is opposite type (p or n type) of compound semiconductor, and diffusion at high temperature are used for fabrication of the LED. In addition, the configuration of the upper electrode of the LED is impractical to be employed as a light source in the optical data-link system for low coupling efficiency. Furthermore, high temperature treatment degrades the optical characteristics due to dopant diffusion and also crystal defect.
The Japanese Unexamined Patent Publication No. Heisei 4-174567 discloses a high brightness LED array to be employed in a printer. The disclosed conventional surface emission type light emitting diode array has a plurality of surface emission type light emitting diodes extracting a light generated in a plurality of active layers provided on a common substrate, through light extracting surfaces formed at opposite side of the substrate. Also, a light reflector layer of semiconductor multiple layers are provided. The light reflector layers are respectively provided in between active layer of surface emission type light emitting diode and the common substrate for reflecting the light by light wave interference. In such prior art, a bottom distributed bragg reflector layer (DBR) is used for reflecting the light back from the substrate. In this case, the same idea for fabricating the surface emission laser is implemented. For the DBR, pairs of GaAs/AlAs is used for reflecting a 880 nm wavelength light toward the substrate. The types and number of pairs to be used in the DBR should be dependent on the output emission wavelength. Also, the same DBR cannot be used in the visible LED where 600 to 650 nm wavelength are concerned.
Namely, the disclosed LED array can be used in the printer but cannot be employed in the data link system. This is because the configuration of the upper electrode is the same as that of the conventional LED as set forth above. The coupling efficiency with a fiber becomes quite low as such LED is employed.
Japanese Unexamined Patent Publication No. Heisei 4-259263 discloses a visible LED employing InAlGaP. The disclosed conventional semiconductor light emitting element is fabricated by sequentially growing n-type clad layer formed of InGaAlP type material, active layer and p-type clad layer, on an n-type GaAs substrate to form a double heterostructure portion. A p-type intermediate band gap layer is formed on the double heterostructure portion, and a p-type contact layer is selectively formed on the intermediate band gap layer. The semiconductor light emitting element is consisted of an active layer formed with an ordered layer having natural super lattice, and the intermediate band gap layer 15 formed with a non-ordered layer, in which the natural super lattice is extinguished by Zn diffusion. By making the band gap of the intermediate band gap layer greater than the band gap of the active layer, the light from the light emitting region can be extracted without blocking by the electrode at the light extracting side. In the disclosed prior art, an additional layer of InGaP, band gap wider than the layer used in active region is used in order to avoid Zn diffusion into the active region. No additional layer for current spreading is used. As set forth above, this type of the visible LED with centrally located circular shaped electrode can not be used in the short distance data link system.
As explained in the above, the LED structure so far proposed, has light output and coupling efficiency, not enough for using in the short distance data link system, especially where POF based system are concerned. This is because, conventional LED has low output power and also low coupling efficiency with the POF. Therefore, it would be highly desirable to design a high brightness visible LED which could be fabricated in low cost and suitable for mass scale production.